The present invention relates to plasma etching. More particularly, the present invention relates to plasma etching of an oxide layer to form high-aspect ratio (HAR) openings.
The use of a device to generate a plasma in a vacuum chamber to etch an oxide layer on a substrate is known. In such oxide etches, a fluorine containing gas may be used to increase etching rate an a carbon monoxide (CO) is used to scavenge fluorine to facilitate discussion, FIG. 1 is a schematic view of part of a wafer 100 with a photoresist mask 102, over a silicon oxide layer 104, over a substrate 106. With reference to figures herein, it should be noted that other additional layers above, below, or between the layers shown may be present. Further, not all of the shown layers need necessarily be present and some or all may be substituted by other different layers. The wafer 100 may be placed in a plasma processing chamber where a medium density plasma at a pressure of greater than 100 milliTorr (mTorr) comprising a fluorine containing gas would be used to etch parts of the oxide layer 104 exposed by the photoresist mask 102. Sometimes CO is added. CO is excited in the plasma and reacts with free Fluorine to form COFx, thus reducing both oxide and photoresist etch rates. Generally the medium density plasma might have a CO flow rate of less than 200 sccm. Fluorine in the medium density plasma would increase not only the oxide etch rate, but will also increase the photoresist etch rate, which will increase the critical dimension (CD) of the openings. A polymer providing gas or carbon monoxide (CO) may be added to increase oxide to photoresist selectivity, but may also deposit within the openings causing etch stop, thus limiting the depth of the openings. Therefore, in the prior art, too much fluorine would reduce oxide to photoresist selectivity and too much CO or polymer providing gas would cause excessive fluorocarbon deposition and result in etch stop.
In addition, oxide etching processes may etch larger openings faster than smaller openings because of an effect called RIE Lag. This usually causes the smaller, higher aspect ratio, openings to etch more slowly than the larger, lower aspect ratio, openings.
It was believed a flow rate of CO above 200 sccm would cause reverse RIE Lag which was believed should be avoided. It was believed that a high flow rate of CO would cause etch stop, which would limit the depth of the openings.
In view of the foregoing, it is desirable to provide an oxide etching process with high oxide to photoresist selectivity without causing etch stop and CD enlargement.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a plasma etch of a layer of a wafer is performed in a low-pressure, high-density, inductively-coupled plasma. A wafer is placed in a processing chamber. A source gas, comprising a fluorine containing gas and carbon monoxide, is flowed into the processing chamber. A plasma is ignited. More than about 0.5% of the carbon monoxide is ionized into CO+ along with excitation of the some of the remaining CO (CO*). The CO* remains in the neutral phase. The CO* reacts with free Fluorine to form COFx while the CO+ contributes oxygen to the etch front and reduces the polymer layer thickness at the etch front. The layer is then etched.
The invention also provides a processing chamber for etching a layer of a wafer. A chamber top with a chamber wall extending from the chamber top to form a cavity is provided. A chuck supports the wafer within the cavity. A gas source provides a fluorine containing gas and carbon monoxide into the processing chamber. An exhaust device is able to maintain pressure in the processing chamber below 40 mTorr. A plasma ignition device is able to ionize more than about 0.5% of the carbon monoxide into CO+.
These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures.